Risk taking behavior peaks during adolescence and can lead to undesired behaviors such as drug use and unsafe sexual activity. Immaturity of the neural circuitry that controls motivated behavior may contribute to adolescent risk taking. Serotonin modulates this neural circuit and may be important for adolescent risk taking because it has been shown to facilitate behavioral inhibition in response to aversive outcomes. However, few studies have investigated the role of serotonin in adolescent risk taking. In preliminary studies we tested the behavioral effects of serotonergic drugs in adult male 67-73 day old (PN67-73) and early adolescent male (PN28-34) rats. We used rodent anxiety tests because behavior in these tests may reflect behavioral inhibition and risk taking as well as anxiety. Depletion of serotonin with the tryptophan hydroxylase inhibitor p- chlorophenylalanine (PCPA) was anxiolytic in adult rats in the novelty induced hypophagia (NIH) test, but had no effect on anxiety in adolescent rats. Adolescent rats were also less sensitive than adults to the anxiogenic effects of the serotonin releaser fenfluramine (2 mg/kg) and the serotonin reuptake inhibitor fluoxetine (10 mg/kg) in the light/dark (LD) test. Preliminary microdialysis in the prefrontal cortex showed that adolescent rats have a lower serotonergic response to fenfluramine (1, 2.5, and 10 mg/kg) than adults. We hypothesize that adolescents have immature serotonergic innervation to the forebrain that results in disinhibited behavior. We will investigate this hypothesis with three specific aims using PN28-32 and PN67-73 male rats. In aim one we will test the impact of serotonin tone and stores by evaluating the effects of fenfluramine (1, 2.5, and 10 mg/kg), fluoxetine (5, 10, and 20 mg/kg), and PCPA (2 doses of 150 mg/kg) in the elevated plus maze (EPM). Aim two will use microdialysis to investigate age differences in serotonergic function in the medial prefrontal cortex, a region in which serotonin modulates anxiety-like behavior and impulsivity. We will use the zero net flux method to compare baseline serotonin levels in adult and adolescent rats. We will complete our preliminary fenfluramine study and use potassium depolarization to assess the capacity to release serotonin. We will then measure the response to a range of doses of fluoxetine (5, 10, and 20 mg/kg) and pretreat animals used in this experiment with saline or 0.3 mg/kg WAY-100635 to compare 5-HT1A regulation of the fluoxetine response. Aim three will determine where in the brain serotonin exerts age-specific effects during performance of an anxiety test. We will compare neuronal activation after performance of the NIH test using expression of the immediate early gene c-Fos in adult and adolescent rats treated with PCPA or vehicle. This proposal is relevant to NIMH strategic objective 1.1 because its goal is to investigate how development of the serotonin system across adolescence changes serotonergic modulation of behavior. This proposal also addresses NIMH strategic objective 2.1 as developmental changes in the serotonergic system could affect the emergence of mood disorders and modulate the effects of drugs used to treat these disorders in adolescents. PUBLIC HEALTH RELEVANCE: This proposal investigates how serotonergic modulation of risk taking behavior may change between adolescence and adulthood. Findings of developmental differences in how serotonin modulates behavior in rodents will increase our understanding of why adolescents engage in negative risk taking behaviors and may also have implications for the emergence and treatment of mood disorders during adolescence.